This blog follows and explains the processes behind interesting geological events. The emphasis is on those events that are energetic, explosive, and have led to, or have the potential to lead to, disasters.

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This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Tuesday, December 6, 2011

Tohoku tsunami IN THE PACIFIC was a "merged tsunami" (Or was it? Why did I put IN THE PACIFIC in capital letters? Read on!)

Left: Ocean heights as observed by two satellites.
Top: at 7:30 hours; Bottom, at 8:20 hours
Right: Computer simulations (black lines) and data
(red and purple lines) on the form of the tsunami.
NASA/JPL-Caltech/Ohio State University
The NASA press release is here.

The Fall meeting of the American Geophysical Union is in full swing and press releases are highlighting some interesting papers. One of these is on so-called "merged tsunamis," which I'll explain in a minute. However, if you Google "merged tsunamis" today, you'll find headlines like:

"Japan was hit by a tsunami formed from TWO giant waves, reveal scientists" (Daily Mail, UK) (OOOPs, note added: one commenter pointed out that the Daily Mail should not be considered a mainstream newspaper...)

"Tsunami that struck Japan in March resulted from merging waves" (CNN, International)

Even academic publications:

"Merging tsunami" doubled destructive power along Japanese coast" (Environment360, from Yale.edu)

Many of the articles are accompanied by photos of the devastation on the coast of Japan.

But, wait a minute!! Here's the actual NASA/JPL news release. While the headline "NASA finds Japan tsunami waves merged, doubling power," might lead you to think that scientists are saying "The tsunami that hit Japan was caused by merged tsunami waves, doubling the power...", that is, in fact, not what the text of the article, nor the accompanying images show. If you look at the images shown on this post (which are the images in the press release) carefully, Japan is in the far upper left corner and the waves that were observed and are modeled are far out away from Japan in the Pacific Ocean. They were observed 7:30 and 8:20 hours AFTER the earthquake. In contrast, the waves that devastated northern Honshu struck in 20 minutes.

Unfortunately, I am not at AGU to hear the paper (which is not being given until Friday morning), but I find the press release to have very little content--it basically says that two satellites captured the above two images, that there was a "merged tsunami," and that this merging phenomenon may account for unexpected destructive power." And, I find the images to be baffling....what do the three black arrows point to? What is the purple line that runs up through the bottom image, and why is it red in the top image? What is the red arrow on the bottom of each image and why has it changed position? What am I supposed to be seeing in these images? The abstract of the actual paper (by Y. Tony Song and others) has a different figure). For info, I have attached the actual abstract at the bottom of this post.

Here's what I do see--in both images the red areas show water that is higher than an arbitrary zero-level (see the scale on the left image). The blue areas, in contrast, represent water that is below the zero level. These two areas correspond to the highest and lowest peaks in the model and data shown on the right side of the figure. All that I can pull out of the two images on the left side is that there isn't as much red or as much blue in the bottom image as in the top one--that is, the high water is less high, and the low water is less deep, which is what you expect as a tsunami spreads out to cover more and more area. My concept of a "merged tsunami" is that two high waves catch up with each other producing a bigger wave by constructive interference. I can't see that in these images.

Readers--HELP!! (And they did, see reader comments!)

And, JPL--shame on you for an ambiguous, if not downright misleading, headline. It did its job in attracting a lot of attention, but it created a lot of misinformation, and that's not the job of a scientific press release.

ABSTRACT BODY: Tsunamis often severely devastate some coastal areas while leaving others with little damage. This unpredictable situation has been a major challenge for accurate and timely tsunami forecasting for evacuating coastal communities. Here we show evidence from satellite observations of the 2011 Tohoku-Oki earthquake-induced tsunami that sheds light on this issue. Three satellites observed the same tsunami front, and for the first time, one of them recorded a tsunami height about twice as high as that of the other two. Model simulations confirm that the amplified tsunami is one of several jets formed through topographic refraction when tsunamis travel across ocean ridges and seamount chains. This process causes the tsunami front to merge as it propagates, resulting in doubling its wave height and destructive potential in certain directions before reaching shore. We conclude that the potential of tsunami merging jets should be taken into consideration for designing coastal tsunami hazard maps and assessing risk levels at coastal oil refineries and nuclear power facilities.
http://science.jpl.nasa.gov/people/Song/

3 comments:

I'm not at AGU either, but to me the pictures above (along with the press release and abstract) seem to suggest the tsunami was diffracted by topography (before the pictures), and then there was constructive interference leading to larger waves. The black arrows appear to point to those areas of constructive interference.

PS Please don't call the Daily Mail a proper newspaper. I would expect nothing less than a headline that incorrect...

I also figured that the new release said nothing new, except for observing merging waves in the tsunami wave train; I asked a modeling colleague Frank Gonzalez for a comment:

"The tsunami amplification mechanisms mentioned are nothing new ... it's well known and well-documented that ridges actas wave guides to trap and direct energy and that two waves can add together to create a larger wave. But it's a prettygarbled and confusing write-up ... full of cute, catchy phrases and lacking a coherent description of what thescientists actually saw ... no doubt the effect of the press release being written by a non-technical author."

I give the scientists less credit --that is, I am not sure it's the news writer's responsibility, I think NASA was going hard for the "big splash" irrespective of it being misleading... I am more cynical than Frank

I went to the talk this morning, so will summarise my understanding. The presenter noted that it was observed more than 20 years ago that the passage of tsunamis across the deep ocean is affected by what he called tsunami jets - i.e. areas of increased tsunami heights. These jets sometimes interact, producing constructive interference and thus higher wave heights. He acknowledged that was nothing new.

Three satellites passed over the ocean basin as the Tohuku earthquake tsunami was transitioning across the Pacific. Two measured peak water heights of about 30 cm, but one (the second in the sequence) measured 60 cm peak heights. The authors compared this observation with their tsunami model and found that there was strong correlation in both the location and the magnitude of this effect, explained by the positive interference effect.

In effect the data is verifying both the idea of positive interference causing local higher water heights, and also the model of that process.

In the final part of the talk the presenter explained that the reason for these tsunami jets is sea floor topography, which means that it is possible to model these effects. His argument was that considering these impacts is important for hazard assessment for tsunamis.

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